The present invention relates to a semiconductor switching device with a self turnoff function.
A conventional gate turnoff thyristor (GTO) and an insulating gate transistor will be used for description of the switching device with a self turnoff function.
In FIG. 1, there is shown a structure of the GTO as a first example of the conventional switching device. As shown, the GTO has a PNPN structure in which a P type anode emitter P1, an N type base N1, a P type base P2, and an N type cathode emitter N2 are successively multilayered. An anode electrode A is connected to the anode emitter P1 through an ohmic contact. Between a cathode electrode K and the cathode emitter N2 and a gate electrode G and the base P2 are also formed ohmic contacts. In operation of the switching device thus structured, a plus voltage is applied to the anode electrode A, and a minus voltage, to the cathode electrode K. Under this condition, if a voltage, of which polarity is positive with respect to the cathode, is applied to the gate electrode G, the GTO is turned on. If a voltage, of which the polarity is negative with respect to the cathode, is applied to the gate electrode G, it is turned off.
FIG. 2 shows a structure of an insulating gate transistor as a second example of the conventional switching device. This transistor also has the PNPN structure, like the GTO. An anode electrode A is coupled with an anode emitter P1 through an ohmic contact. A cathode electrode K is also coupled with a cathode emitter N2 through an ohmic contact. A gate electrode G is formed over a base N1, a base P2 and the cathode emitter N2, with an insulating layer 2 formed therebetween. In operation of the switching device with such a structure, a plus voltage is applied to the anode electrode A and a minus voltage is applied to the cathode electrode K. Under this condition, if a voltage which is positive in polarity with respect to the cathode, is applied to the gate electrode G, an N channel is formed in the cathode base P2, resulting in turning on of the transistor. Then, if the voltage applied to the gate electrode G becomes zero, the N channel disappears and the transistor is turned off.
The conventional switching devices as mentioned above have the following disadvantages.
In the case of the first example of FIG. 1, the switching device is turned on in a current control manner in which current (gate current) is fed to the gate. This turn-on operation depends largely on majority and minority carriers, and further a spreading rate of plasma. In this respect, the turnoff characteristic of this device is not satisfactory.
In the case of the second example of FIG. 2, the switching device is turned on in a voltage control manner in which a channel is formed in the gate. Therefore, this device has a better turn-on characteristic than the conventional switching device of which the turn-on operation depends on the carrier diffusion. The switching device of the second example is turned off by erasing the channel, not by forcibly extracting carriers or holes from the gate, unlike the first example. Accordingly, a number of excess carriers are still left inside the switching device immediately after it is turned off. The excess carriers flow in the form of current (tail current) after it is turned off. This current deteriorates the turnoff characteristic.
Further, in the second example, to turn off the switching device, the application of the gate voltage is stopped. To keep the on-state of the device, voltage must be continuously applied to the gate. This means that when it is used as a self turnoff type switching device, only possible operation mode of the switching device is the transistor mode.